An automobile generator is a combination of a multi-phase claw-pole alternator and a rectifier. The alternator includes a rotor with a coil to provide exciting magnetic flux, and a stator with multiple phase windings for AC output currents. The rectifier is used to convert the AC currents to DC out current to charge the battery and support other electrical equipment within a vehicle. Almost all automobile alternators are claw-pole type alternators for their low rotor manufacturing cost, good power density, and high reliability.
In a vehicle, the alternator is driven directly by the engine accessory belt, and the alternator output current increases along with increases in engine speed. The most important criteria of an automotive alternator is DC output current at engine idle speed (about 550 rpm corresponding to an alternator speed of about 1600 rpm).
In an alternator, there is an air gap between the rotor and the stator to allow the moveable rotor to spin without touching the static stator core. This air gap has certain reluctance for magnetic flux. To maximize the output current of an alternator, the air gap should be designed as small as possible. The nominal air gap in a typical alternator is approximately 0.4 mm, however, the airgap can be significantly larger or smaller. When the air gap is reduced the output current is substantially improved.
However, the air gap of a claw-pole alternator should not be designed too small. When running at very high speeds, the alternator rotor poles will deflect due to centrifugal forces and the pole tip will touch the stator. Typically, the spin speed at which the deflected rotor pole fingers start to reach the stator inner diameter is the maximum speed limit of the alternator. Therefore, there is a minimum air gap limit in a claw-pole alternator. This minimum air gap equals the maximum rotor pole centrifugal deflection plus the manufacturing tolerance of the stator inside diameter and the rotor outside diameter. The designed air gap should not be smaller than the minimum air gap limit to prevent interference between the rotor and the stator at the designed maximum speed. Typically, the maximum speed is from 18,000 rpm to 22,000 rpm for most alternators on the market.
In recent years, more and more electrical and electronic loading has been added in vehicles. Automobile OEMs are requiring that future alternators provide much higher electrical power output than today's alternators and require that future alternators be the same size, or even smaller, than today's alternators. Therefore, a need exists for an improved alternator, which can withstand operating speeds on the order of 25,000 rpm without the risk of interference between the rotor assembly and the stator assembly. It is preferable that these machines have relatively small air gaps between the rotor and stator in order in achieve the power density required.